This invention relates to a solution of ethyllithium in dibutyl ether.
Ethyllithium (EtLi) is one of the alkyl lithiums. These are used as starters in anionic polymerisation and in preparative organic synthesis. There they are employed as alkylating agents or, because of their strongly basic character, are used for producing carbanions, which can be reacted with a multitude of electrophilic reagents. (Houben-Weyl, Methoden der Organischen Chemie, Thieme Verlag, Volume XIII/1, 1970; Wakefield, B. J., The Chemistry of Organolithium Compounds, Pergamon Press, London, 1974; Organolithium Methods, Academic Press, London, 1988; Brandsma, L., Preparative Polar Organometallic Chemistry, Vol. 1, 1987, and Vol. 2, 1990, Springer Verlag; M. Schlosser et al., Organometallics in Synthesis, 1994, John Wiley and Sons, Sussex).
The preparation of alkyl lithiums according to the equation
2 Li+Rxe2x80x94Halxe2x86x92Rxe2x80x94Li+Lixe2x80x94Hal 
goes back to K. Ziegler (K. Ziegler, H. Colonius, Liebigs Ann. Chem. 476, 135, 1930). The synthesis can be carried out both in hydrocarbons (Gilman, H. et al., JACS, 63, 2479, 1941) and in ethers (Gilman H., JACS, 71, 1499, 1949).
The use of ethers as solvents is extremely limited here because, with the exception of the case of methyllithium in diethyl ether, the alkyl lithium attacks the ether, with the resulting decomposition of the alkyl lithium and cleavage of the ether. (A. Maercker, Ang. Chem. 99, 1002, 1987). For ethyllithium in diethyl ether, the half life period was found to be 54 hours at 25xc2x0 C. and 17 hours at 35xc2x0 C. (T. L. Brown et al., JACS, 88, 2174, 1966). In the case of ethyllithium in tetrahydrofuran (THF), the decomposition of THF by ethyllithium with the formation of ethene and Li ethenolate already takes place at xe2x88x9220xc2x0 C. (A. Rembaum et al., J. Polym. Sci. 56, p. 17, 1962).
Ethereal solutions of ethyllithium are produced in situ as required. A commercial preparation and provision of these solutions has not hitherto been possible.
The possibility of using hydrocarbons as solvents for ethyllithium has hitherto been extremely limited, because the solubility of ethyllithium is very low owing to the small alkyl group (solubility in hexane: 2%, in toluene: 4%, in benzene: 5%) and such dilute solutions are uneconomic. Furthermore, a solution of ethyllithium in toluene has the disadvantage that toluene is metalated by ethyllithium. Benzene is hardly suitable as a solvent because of its carcinogenic properties.
Attempts have been made to bring EtLi into solution by complexing with other Li-alkyls, with the aim of producing more highly concentrated ethyllithium solutions (Weiner, West, JACS, 85, 4856, 1963). A disadvantage of such solutions is the presence of other alkyl groups, which make these solutions unsuitable for the introduction of the ethyl group. There have also been attempts to complex EtLi with Li ethoxide and thus to stabilise it (Brown, T. L., JACS, 88, 2174, 1966). Here the altered reactivity of the EtLi is a disadvantage.
The object of the present invention is to eliminate the disadvantages of prior art and, in particular, to provide a relatively stable solution of ethyllithium, without other alkyl groups, having an EtLi concentration of up to 15% (% data, unless indicated otherwise, are wt. %). A further object is to provide a process for preparing such an EtLi solution.
This object is achieved by an EtLi solution in dibutyl ether and by a process for preparing such a solution, in which process lithium in dibutyl ether is reacted with ethyl chloride to form ethyllithium.
Surprisingly, it has been found that the EtLi solution according to the invention is relatively stable (see Table 1).
For the loss of active base, a first-order reaction is assumed; for this k=[end concentration(active base)/initial concentration(active base)]xc3x97100/storage time, in days.
The solution of ethyllithium in dibutyl ether is prepared by Ziegler""s method. Here the lithium, for example, in the form of a dispersion in dibutyl ether, is placed in a vessel and reacted by introducing gaseous ethyl chloride. In the case of an approximately 90% conversion, a solution having a content of up to 12% ethyllithium is obtained after filtration. The prepared solution having a maximum content of 12% ethyllithium can be concentrated to 15% by evaporation to small volume under vacuum at room temperature.
Owing to the pyrophoric behaviour of these highly concentrated solutions, it is advisable to adjust commercial solutions to an ethyllithium content of 7 to 8%. This can be effected by directly synthesising a 7 to 8% solution or by diluting a more highly concentrated solution to 7 to 8%. The diluents used can be dibutyl ether, or hydrocarbons (cyclic or acyclic) having 5 to 12 C atoms or aromatic hydrocarbons having 6 to 12 C atoms. These solutions are then no longer pyrophoric. Solutions of ethyllithium in dibutyl ether and in one or more hydrocarbons contain preferably 5 to 8% ethyllithium.
Lithium is used preferably as granules (for example, having a particle size of up to 3 mm) and particularly preferably in pulverous form (particle size 10 to 500 xcexcm, preferably 10 to 200 xcexcm), with an excess of lithium generally being used. A stoichiometric quantity of 2 mol Li and in addition an excess of 0.02 to 0.5 mol, preferably 0.02 to 0.3 mol, lithium is used per mol ethyl chloride.
The initial concentration is preferably so chosen that the product concentration in the final solution is approximately 8%. As the reaction of lithium with ethyl chloride is exothermic, the reactants are preferably cooled during the conversion in order to trap the heat of reaction and to avoid secondary reactions and decomposition of the product. The maximum reaction temperature is 40xc2x0 C. A reaction temperature of xe2x88x9210xc2x0 C. to +10xc2x0 C. is preferred. The feed rate is so chosen that the heat of reaction is trapped efficiently and excess ethyl chloride does not escape from the reaction mixture. An excessively rapid metering of the ethyl chloride should be avoided, owing to its solubility in dibutyl ether and a possible Wurtz reaction. The lithium chloride formed during the reaction is filtered off at the end of the reaction; the filtered solution is the final product solution or may, as required, be diluted to the desired concentration using dibutyl ether or a hydrocarbon.
The preferred storage temperature of the solution is xe2x88x9220xc2x0 C. to +5xc2x0 C.; a storage temperature of 0xc2x0 C. is particularly preferred.
The subject matter of the invention is explained in more detail below by means of the following Examples.